This invention relates to a method and a measuring instrument for measuring, in particular, the flow rate of flowing media using the Coriolis principle, having at least one measuring the conducting the flowing medium, at least one vibrator which sets the measuring tube vibrating, and having at least two sensors which detect the tube movement and an electronic evaluation system which processes the measuring signals.
Coriolis mass flow meters for flowing media are widespread, and are known in very varied embodiments.
In general, in the case of a Coriolis mass flow meter at least one measuring tube which is symmetrical relative to its geometrical midpoint is set vibrating by a vibrator. The vibration of the measuring tube is monitored in this case by a control circuit which receives its actual value information from at least one of the above-named sensors which detect the tube movement.
The vibration of the measuring tube causes the Coriolis force in the flowing medium. The Coriolis force oscillates with the frequency of the tube vibration. It causes an additional movement which is superimposed on the tube movement excited by the vibrator, and leads to a small, phase-shifted movement of the tube sections about the geometrical midpoint of the measuring tube. This phase shift represents the measuring effect. It is proportional to the Coriolis force and, therefore, proportional to the flowing mass. The tube movement is detected by two sensors, arranged in general symmetrically relative to the tube midpoint, and is fed to the electronic measurement system for evaluation.
The Coriolis force is defined as (complex phasors are represented by an underscore):
Fc=xe2x88x922m(xcfx89xc3x97v)
where xcfx89=angular velocity of the measuring tube and v=rate of flow of the liquid in the measuring tube.
Since the angular velocity is a function of the tube amplitude, the measuring signal is also a function of the amplitude. Consequently, the evaluation is standardized to the tube amplitude, that is to say m=measuring signal/tube amplitude. In component nomenclature, the two sensor signals are as follows for the abovementioned symmetrical processes:       A    _    =                    R        _            +                        Fc          _                ⁢                  xe2x80x83                ⁢        and        ⁢                  xe2x80x83                ⁢                  B          _                      =                  R        _            -              Fc        _            
where R=tube amplitude and Fc=Coriolis amplitude.
By addition, this yields twice the tube amplitude
A+B=2R
and by subtraction twice the Coriolis amplitude
Axe2x88x92B=2Fc
at the position of the sensors. If asymmetrical signal interference now occurs in the case of which individual signal components, or else both components of the sensors change, this leads to measuring errors and control problems.
The geometrical midpoint of the measuring tube is free from additional movements caused by the Coriolis force. In many embodiments of mass flow meters, it is the fastening point of the above-named vibrator.
The installation of the vibrating system of the xe2x80x9cCoriolis mass flow meterxe2x80x9d in a piping system couples the mass flow meter to the environment vibrationally. Consequently, it is possible to excite both structural and hydraulic vibrations at the operating frequency of the mass flow meter which cause reactions in the mass flow meter and are not detected by the electronic measurement system. The interactions of these vibrations generally occur asymmetrically and can lead to substantial disturbances of the measuring instrument. A possible consequence of these interactions are instabilities in the control loop, that is to say functional disturbances can occur.
The most unfavorable consequence of these interactions are substantial disturbances of the metrological properties. The cause of this is static and dynamic zero shifts.
Static zero shifts, that is to say apparent measuring signals in the case of a zero flow rate, are usually compensated by means of the electronic measurement system. Since they feature in the flow rate calculation as an offset, the static zero shifts lead to large measurement value deviations, particularly in the lower flow rate range of the mass flow meter.
Here, dynamic zero shifts are the additional shifts, not constant as a rule, occurring during throughflow owing to parameter variation in the installation. They are characterized by the fact that, as variables which are dependent on all possible process parameters and have not so far been capable of identification, quantification and compensation, they result in measured value deviations, which have not so far been correctable, over the entire measuring range, and additionally lead to problems in reproducibility.
Starting from these problems, it is the object of the invention with regard to a method and a mass flow instrument to achieve compensation of the static and dynamic interference on the operational and metrological response independently of the measuring tube design, and to render the overall system insusceptible to interference.
The invention is distinguished in that in addition to the two sensors arranged symmetrically relative to the site of vibration, a further, third sensor is arranged at the site of vibration generation on an arbitrarily shaped measuring tube. The result is to produce a spatially fixed point via which the reference position of the deflections occurring at the two other sensors is clearly defined.
The absolute value of the added amplitudes A+B such as is frequently used for amplitude control depends on the phase shift caused by the Coriolis force.
By contrast, the third sensor according to the invention supplies as further measuring signal the pure tube amplitude, and thus an actual-value signal, independent of flow rate and interference, for the control circuit. By means of this additional signal, the two symmetrically arranged sensors are used to calculate the mass flow from the signal of respectively one of these sensors and the tube amplitude signal of the third sensor, that is to say the mass flow is calculated 2 times. The two flow rates determined by means of the third sensor correspond in this case to the real processes around the geometrical midpoint of the measuring tube. The sum of the two flow rates yields the flow rate, as indicated above. The two flow rates determined are equal, since in the normal operating state the signals are symmetrical, as stated above. Should instances of interference arisexe2x80x94these being generally asymmetrical, as stated abovexe2x80x94the additionally determined flow rate become asymmetrical, the direction of the interference becoming detectable.
The sum likewise changes, that is to say the instrument measures wrongly owing to the interference. They symmetry is restored and the interference is eliminated by continuously comparing the calculated flow rate and using appropriate correction algorithms to correct the signal side influenced by interference.
This achieves the object of eliminating interference from the metrological response.
A further subaspect of the object, which is intended to improve the operational response, is achieved by two special configurational features according to the invention. Redundancy is provided by the two-fold determination of the flow rate. In the case of severe interference or a failure of a sensor, an evaluation section can be used to maintain full measurement capacity. However, elimination of interference no longer obtains. The control response is improved by virtue of the fact that an actual-value signal is available which is not influenced by the flow rate. A further important aspect of the invention resides in that the failure (overshooting of the previously fixed tolerance limit) of a sensor is automatically detected by the measuring transducer, and the detection of measured values is altered in such a way that the instrument continues to operate acceptably.
The user is advised by the software (display or communication) of the failed sensor, and can institute repair measures if appropriate. These need not be performed immediately, but whenever the process allows. The probability of a total failure of the measurement system can thereby be substantially reduced. The system is, as it were, of redundant design.
The mode of operation of the invention is represented by way of example below. To explain the problems which have been solved, the zero-point compensation is briefly derived formally further below with the aid of the 3 sensors arranged according to the invention.
However, firstly the redundant mode of operation and/or redundant design of the method and the device is described:
In order to set the measuring tube vibrating mechanically, an electromagnetic device is used to apply a force to the measuring tube (sensor/driver C or sensor/driver D of FIG. 9). The force is proportional to the current through the driver coil. The measuring tube is a resonant circuit with a quality Q.
The driver current required to generate a mechanical vibration amplitude A is a function of the quality, that is to say the losses of the mechanical resonant circuit. The highest quality is to be found in the case of the non-installed, empty pickup, that is to say the driver current is lowest. This reference value is measured and stored.
After installation of the pickup and filling with a medium, the damping of the resonant circuit can be increased, that is to say the driver current increases. This current is also measured and stored. A system zero point compensation is likewise carried out in this operating state, that is to say the flow rate indication is set to zero. Should operating states now arise in which vibrational energy is transferred to the outside by hydraulic or mechanical coupling (for example in the case of gas-containing media or gas bubbles in the installation), the damping of the mechanical resonant circuit increases, and therefore the driver current increases.
Feedback of the dissipated vibrational energy produces a shift in the dynamic zero point which is proportional to the driver current.
Consequently, it is possible to use the continuously measured driver current to achieve the absolute value of the dynamic system zero point. The sign is determined with the aid of the third sensor.
Furthermore, a damped mechanical resonant circuit has a slightly different frequency than an undamped one. This affects the density measurement, since the density of the medium located in the pickup is calculated from the resonant frequency of the pickup. It is possible to use the measured driver current to compensate this frequency shift.
Consequently, it is possible to use the third sensor and a measurement of the driver current to compensate a dynamic zero shift and a density shift on the basis of a varying damping of the pickup resonant circuit.